Engine systems may be configured with boosting devices, such as turbochargers or superchargers, for providing a boosted aircharge and improving peak power outputs. The use of a compressor allows a smaller displacement engine to provide as much power as a larger displacement engine, but with additional fuel economy benefits. However, compressors are prone to surge. For example, when an operator tips-out of an accelerator pedal, an engine intake throttle closes, leading to reduced forward flow through the compressor, degrading turbocharger performance and possibly leading to compressor surge. Compressor surge can lead to NVH issues such as undesirable noise from the engine intake system.
One example attempt to address compressor surge is shown by Styles et al. in US 20150047346. Therein, when the engine operates at or beyond a compressor surge line, a continuously variable compressor recirculation valve (CCRV) is opened to recirculate a portion of cooled boosted air from downstream of a charge air cooler to the compressor inlet. The approach improves the compressor pressure ratio, mitigating surge.
The inventors herein have identified an approach that may augment the CCRV and/or reduce the need for a CCRV in systems with compound boosting, leading to potential for improved performance and/or cost reduction. The method includes: bypassing a second compressor and providing a flow of compressed air to a piston engine via a first compressor in normal operating conditions; and in response to a decrease in demanded engine torque, accelerating the second compressor and reducing the flow of compressed air to the engine. In this way, a second compressor staged upstream of a first compressor can be used to mitigate surge.
As one example, a boosted engine system may include an electric supercharger coupled upstream of a turbocharger. During conditions when boost is required and while the turbine of the turbocharger is spinning up, the electric supercharger may be used to provide compressed air to the engine. Then, once the turbine spins up, the turbocharger compressor may be used to provide compressed air to the engine, while bypassing the supercharger. In response to throttle reduction, for example in response to an operator pedal tip-out, to reduce the likelihood of surge at the turbocharger compressor, the electric supercharger may be spun up while the turbocharger continues to spin (due to high inertia). This results in an increase in pressure at the turbocharger compressor inlet, moving the compressor ratio of the turbocharger further from a surge threshold. Once the turbocharger flow has been sufficiently reduced, the supercharger may be disabled, and compressed air may once again be provided via the turbocharger, when needed.
Compressor surge boundary is defined in terms of compressor flow and pressure ratio. CCRV initial impact is in the flow domain, while the approach defined herein has initial effect in the pressure ratio domain. Combining the two approaches extends the surge free operating regime, improving system performance, and/or reduces the size of the CCRV, which lowers system cost. In some cases, the CCRV may be eliminated and the approach described herein may be combined with a conventional on/off CRV, which also reduces system cost.
The technical effect of increasing the compressor pressure ratio at the first, downstream compressor via operation of the second, upstream compressor is that surge can be reduced without degrading boosted engine performance. By operating the supercharger to increase pressure upstream of the first compressor's inlet, and decrease flow through the first compressor, the need for operating a compressor recirculation valve to mitigate surge is reduced. As such, this improves the performance of a boosted engine system having staged charging devices.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.